Mammalian and other eukaryotic MSRs have been recently closed and over-expressed to facilitate structure-function studies. We have established a collaboration with Drs. Jacob Moskovitz and Earl Stadtman at the National Institutes of Health towards solving the structure of both human and yeast MSRs. The yeast enzyme contains 183 amino acids (Mr 21,000) and monomeric in solution. It also corresponds to the minimal domain required for MSR activity. Considering the variety of functions mediated by MSR it is critical to understand the structural aspects of this physiologically important system.